Once more the party halted. Shut in as they were by the high, close walls, the sound of the waterfall came to them now only as a dull rumble; but when they spoke it was in whispers. Apart from the risk of being heard by an unseen enemy, there was an atmosphere of mystery and awesomeness that weighed oppressively on their minds.
"What are we to do?" asked Jackson.
"Go on!" Forrester replied, firmly. "We can hardly be seen. The sides are so smooth and straight that no one could perch anywhere to molest us, except at the corners. We must be on our guard there."
"But surely no one can live here! Nothing could grow; there doesn't even appear to be moss on the rock, and the air's as stuffy as in a cave."
"Man, don't argufy!" said Mackenzie. "Straight ahead!"
They continued their course. Every now and again the rift turned sharply to one side or the other, and the smooth floor, unimpeded by loose rocks or boulders, always ascended, more and more steeply as they advanced. Strangely enough, the higher they went the stuffier the air became, and the deeper their sense of oppression, or rather, perhaps, of nervous strain. Mackenzie, who had once been down a coal-mine in Lanarkshire, suspected the presence of poisonous gases.
"There can't be fire-damp," he murmured, "but it may be carbonic acid. Bide a wee while I strike a match."
But this fear was dispelled when the flame burned brightly for a second or two. He extinguished it abruptly.
"Hoots! I'm an ass!" he said. "Someone may have seen the light; and if there are men about, I'd rather see them first than they us."
"My skin is tingling just as if I'd got a grip of the terminals of a battery," Jackson remarked.